|KOIKE, STEVE - University Of California - Cooperative Extension Service|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/23/2009
Publication Date: 1/15/2010
Citation: Koike, S., Martin, F.N. 2010. First report of Phytophthora root rot, caused by Phytophthora cryptogea, on spinach in California. Plant Disease. 94(1):131.
Interpretive Summary: This disease note describes a new root and crown disease of spinach in the Salinas Valley caused by Phytophthora drechsleri.
Technical Abstract: In 2006 and 2007, commercially grown spinach (Spinacia oleracea) in California’s coastal Salinas Valley (Monterey County) was affected by an unreported root rot disease. Disease was limited to patches along the edges of fields. Affected plants were stunted with chlorotic older leaves. As disease progressed, most of the older foliage first wilted and then turned tan and dry; youngest leaves remained green but were stunted and leathery in texture. Plants most severely affected died. Symptoms on roots were mostly restricted to the distal portion of the root system, where feeder roots and the main taproot turned black. Isolations from root lesions consistently resulted in the recovery of a Phytophthora species. The isolates were heterothallic and based on morphological and cytochrome oxidase 2 gene sequence data (GenBank accession GQ984233) the pathogen was identified as P. cryptogea. To evaluate pathogenicity, individual inocula of four isolates were used to inoculate four-wk-old spinach plants (cv. Bolero). After treatment, pots were placed for 24 hr in shallow trays of water to saturate the root zone, then were removed from trays and incubated in a greenhouse. After 9 days, inoculated plants showed symptoms similar to that observed in the field; after 13 days, roots were examined and found to show the black necrosis as seen in the field. P. cryptogea was isolated from all inoculated plants. To our knowledge, this is the first report of Phytophthora root rot of spinach caused by P. cryptogea in California. This finding is significant because spinach in California is subject to root rots caused by three other pathogens (Fusarium oxysporum, Pythium species, Rhizoctonia solani); symptoms from these root rots are very similar to those caused by P. cryptogea, thereby complicating diagnosis. This pathogen has been documented on spinach in Germany and Sweden.